U.S. patent application number 13/663511 was filed with the patent office on 2014-05-01 for high responsivity device for thermal sensing in a terahertz radiation detector.
This patent application is currently assigned to International Business Machines Corporation. The applicant listed for this patent is International Business Machines Corporation. Invention is credited to Dan Corcos, Danny Elad, Noam Kaminski, Bernhard Klein, Lukas Kull, Thomas Morf.
Application Number | 20140117237 13/663511 |
Document ID | / |
Family ID | 50546145 |
Filed Date | 2014-05-01 |
United States Patent
Application |
20140117237 |
Kind Code |
A1 |
Corcos; Dan ; et
al. |
May 1, 2014 |
HIGH RESPONSIVITY DEVICE FOR THERMAL SENSING IN A TERAHERTZ
RADIATION DETECTOR
Abstract
There is provided a novel and useful a high responsivity device
for thermal sensing in a Terahertz (THz) radiation detector. A load
impedance connected to an antenna heats up due to the incident THz
radiation received by the antenna. The heat generated by the load
impedance is sensed by a thermal sensor such as a transistor. To
increase the responsivity of the sense device without increasing
the thermal mass, the device is located underneath a straight
portion of an antenna arm. The transistor runs substantially the
entire length of the antenna arm alleviating the problem caused by
placing large devices on the side of the antenna and the resulting
large additional thermal mass that must be heated. This boosts the
responsivity of the pixel while retaining an acceptable level of
noise and demanding a dramatically smaller increase in the thermal
time constant.
Inventors: |
Corcos; Dan; (Nesher,
IL) ; Elad; Danny; (Haifa, IL) ; Kaminski;
Noam; (Kiryat Tivon, IL) ; Klein; Bernhard;
(Zurich, SZ) ; Kull; Lukas; (Zurich, SZ) ;
Morf; Thomas; (Grosss, SZ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
International Business Machines Corporation |
Armonk |
NY |
US |
|
|
Assignee: |
International Business Machines
Corporation
Armonk
NY
|
Family ID: |
50546145 |
Appl. No.: |
13/663511 |
Filed: |
October 30, 2012 |
Current U.S.
Class: |
250/338.1 ;
374/121; 438/54 |
Current CPC
Class: |
G01J 5/0837 20130101;
H01Q 21/26 20130101; G01N 21/3581 20130101; G01J 5/20 20130101 |
Class at
Publication: |
250/338.1 ;
374/121; 438/54 |
International
Class: |
G01J 5/20 20060101
G01J005/20 |
Claims
1. A thermal sensor for use in a terahertz (THz) radiation detector
having one or more antenna arms, comprising: an electrical device
adapted to respond to changes in temperature; and wherein said
device is located beneath at least one of said antenna arms of said
detector.
2. The sensor according to claim 1, wherein said electrical device
comprises a transistor.
3. The sensor according to claim 1, wherein said electrical device
comprises a diode.
4. The sensor according to claim 1, wherein said electrical device
comprises a resistor.
5. The sensor according to claim 1, wherein said electrical device
comprises a device whose current (I)-voltage (V) characteristic is
temperature dependent.
6. The sensor according to claim 1, wherein said electrical device
is substantially covered by an antenna arm.
7. The sensor according to claim 1, wherein said electrical device
is under a straight portion of an antenna arm.
8. The sensor according to claim 1, wherein the size of said
electrical device is the longest able to fit under an antenna
arm.
9. The sensor according to claim 1, wherein said electrical device
comprises a transistor whose width substantially fits under an
antenna arm.
10. The sensor according to claim 1, wherein said electrical device
comprises a transistor whose length substantially fits under an
antenna arm.
11. The sensor according to claim 1, further comprising one or more
additional electrical devices, each placed under a different
antenna arm.
12. The sensor according to claim 11, wherein said plurality of
electrical devices are connected in parallel so as to increase
effective device size.
13. A thermal sensor for use in a terahertz (THz) radiation
detector having one or more antenna arms, comprising: a transistor
responsive to changes in temperature; and wherein said device is
located substantially completely beneath an antenna arm of said
detector.
14. The sensor according to claim 13, wherein said electrical
device comprises a transistor whose width substantially fits under
an antenna arm.
15. The sensor according to claim 13, further comprising one or
more additional transistors, each placed under a different antenna
arm.
16. The sensor according to claim 15, wherein said plurality of
transistors are connected in series or parallel so as to increase
their effective size.
17. A method of fabricating a thermal sensor for use in a terahertz
(THz) radiation detector having one or more antenna arms,
comprising: providing a semiconductor substrate; fabricating an
electrical device responsive to changes in temperature on said
substrate along a path of an antenna arm; and fabricating the
conductive portion of said antenna arm directly over said
electrical device such that the width of said electrical device is
the longest that can fit under said antenna.
18. The method according to claim 17, wherein said electrical
device comprises a transistor.
19. The method according to claim 17, wherein said electrical
device comprises a diode.
20. The method sensor according to claim 17, wherein said
electrical device comprises a resistor.
21. The method according to claim 17, wherein said electrical
device comprises a device whose current (I)-voltage (V)
characteristic is temperature dependent.
22. The method according to claim 17, further comprising one or
more additional electrical devices, each placed under a different
antenna arm.
23. The method according to claim 22, wherein said plurality of
electrical devices are connected in parallel so as to increase its
effective size.
24. A detector for detecting terahertz (THz) radiation, comprising:
an antenna having one or more elements supported by a holding arm;
a load impedance directly coupled to said antenna and operative to
convert said received terahertz radiation to thermal energy; and a
thermal sensor operative to generate an electrical signal in
accordance with the heat generated by said load impedance, wherein
said thermal sensor located underneath at least a portion of said
antenna.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to the field of semiconductor
imaging devices, and more particularly relates to a high
responsivity device for thermal sensing in a Terahertz (THz)
radiation detector.
BACKGROUND OF THE INVENTION
[0002] THz radiation imaging is currently an exponentially
developing research area with inherent applications such as THz
security imaging which can reveal weapons hidden behind clothing
from distances of ten meters or more; or medical THz imaging which
can reveal, for example, skin cancer tumors hidden behind the skin
and perform fully safe dental imaging. Constructing prior art THz
detectors is typically a challenging endeavor since both radiation
sources and radiation detectors are complex, difficult and
expensive to make.
[0003] THz radiation is non-ionizing and is therefore fully safe to
humans unlike X-ray radiation. THz imaging for security
applications, for example, uses passive imaging technology, namely
the capabilities of remote THz imaging without using any THz
radiation source thus relying solely on the very low power natural
THz radiation which is normally emitted from any room temperature
body according to well-known black body radiation physics. Passive
THz imaging requires extremely sensitive sensors for remote imaging
of this very low power radiation. Prior art passive THz imaging
utilizes a hybrid technology of superconductor single detectors
cooled to a temperature of about 4 degrees Kelvin which leads to
extremely complex (e.g., only the tuning of the temperature takes
more than 12 hours before any imaging can take place) and expensive
(e.g., $100,000 or more) systems. A detector is desirable that can
be used to detect THz radiation and that has much lower potential
cost compared with existing superconducting solutions. Passive THz
imaging, however, requires three orders of magnitude higher
sensitivity compared with passive infrared (IR) imaging, which is a
challenging gap.
SUMMARY OF THE INVENTION
[0004] There is provided a novel and useful high responsivity
device for thermal sensing in a Terahertz (THz) radiation detector.
A load impedance connected to an antenna heats up due to the
incident THz radiation received by the antenna. The heat generated
by the load impedance is sensed by a thermal sensor such as a
transistor. To increase the responsivity of the sense device
without increasing the thermal mass, the device is located
underneath a straight portion of an antenna arm.
[0005] The transistor thus runs substantially the entire length of
the antenna arm. This alleviates the problem caused by placing
large devices on the side of the antenna in that the additional
area taken up by the sensor transistor translates to a large
additional thermal mass that must be heated by the radiation
signal. By placing the sensor below the antenna, a considerably
smaller "area penalty" is paid. Such a solution allows boosting the
responsivity of the pixel while still retaining an acceptable level
of noise and demanding a dramatically smaller increase in the
thermal time constant.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] The invention is herein described, by way of example only,
with reference to the accompanying drawings, wherein:
[0007] FIG. 1 is a diagram illustrating the structure of an example
bolometer for detecting THz radiation;
[0008] FIG. 2 is a diagram illustrating a first example embodiment
cross dipole antenna incorporating a thermal sensor;
[0009] FIG. 3 is a diagram illustrating an example layout of an
NMOS transistor where the arm of a dipole antenna overlaps the gate
of the transistor over its entire length;
[0010] FIG. 4 is a diagram illustrating a second example embodiment
cross dipole antenna incorporating a thermal sensor located
underneath a dipole arm;
[0011] FIG. 5A is a perspective view of a portion of the detector
of FIG. 4;
[0012] FIG. 5B is a side view of a portion of the detector of FIG.
4;
[0013] FIG. 6A is a diagram illustrating the end portion of a
dipole arm located over the thermal sensor transistor;
[0014] FIG. 6B is a perspective view of the end portion of a dipole
arm located over the thermal sensor transistor;
[0015] FIG. 7A is a diagram illustrating the electrical model of
the detector of FIG. 4; and
[0016] FIG. 7B is the equivalent schematic diagram of the detector
of FIG. 4.
DETAILED DESCRIPTION OF THE INVENTION
[0017] The invention provides a high responsivity device for
thermal sensing in an apparatus for detection of Terahertz (THz)
radiation. The high responsivity device pertains to the field of
Terahertz (THz) wave imaging which is the visualization by
detection of THz radiation being irradiated or reflected from
objects in the imager's field of view. Apart from being a
non-ionizing radiation, the wavelengths of the THz portion of the
electromagnetic (EM) spectrum are able to penetrate through
numerous things such as fog, clothing, packages, etc., enabling
imaging with high resolution even by means of relatively small
radiating aperture which is crucial in space constrained
environments. These qualities make imaging in this part of the
electromagnetic spectrum a prime candidate for use in security,
surveillance, navigation, etc. systems.
[0018] A diagram illustrating the structure of an example bolometer
for detecting THz radiation is shown in FIG. 1. The bolometer,
generally referenced 10, comprises the body under test (BUT) 12,
lens 14, housing 11, pixel array 18 and read out circuitry 16.
[0019] The detection of the THz radiation is performed by an
antenna with a resistive load, directed at a specific pixel on the
body under test (BUT). The antenna converts the THz electromagnetic
energy into electrical current that heats the resistive load. The
temperature change of this resistor is then measured as an
indication of the temperature of the body under test (BUT).
[0020] Sensors at THz frequencies are typically Bolometers, since
electronics cannot reach these frequencies. Bolometers can be
implemented using slightly modified CMOS techniques or using a CMOS
SOI process with MEMS post processing. A THz sensor can be directly
integrated with readout circuitry in a CMOS-SOI process. One sensor
that can be used is an antenna coupled bolometer. The THz sensor is
realized with a temperature dependent resistor or with a FET where
it's strongly temperature dependent subthreshold current is used as
a sensor.
[0021] A bolometer based detector may be coupled with a lens on the
top of the structure to collect the incident electromagnetic energy
of individual pixels. Each pixel is adapted to be thermally
isolated from other pixels and from the entire structure. This is
achieved by creating a vacuum around the bolometers. In order to
increase the sensitivity of the bolometer, the noise is often
reduced by cooling the entire structure and the antenna down to
cryogenic temperatures.
[0022] An imaging device (i.e. an imager or detector) may comprise
a 2D array of elements (i.e. pixels), situated as linear arrays or
even by single elements that are optically or mechanically scanned.
Imaging systems can be either passive (only receiving) or active
(illuminating the target) and usually include suitable optical
components. An uncooled passive THz system is particularly
attractive due to the potentially low manufacturing (and operating)
cost and because it does not involve health-related risks.
[0023] In general terms, the detection process can be decomposed to
following three steps: (1) reception of the incoming radiation
impinging the pixel; (2) conditioning of the electromagnetic signal
(e.g., filtering, amplification, transduction, etc.); and (3) read
out by an electronic circuit.
[0024] The present invention deals with the challenges presented by
the detection of signals having long wavelengths (sub-millimeter
waves) using antenna coupled thermal sensing devices. In
particular, the invention relates to the second step, namely the
efficient transduction of the electromagnetic signal (THz signal)
into a measurable electrical quantity when using a thermal
sensor.
[0025] Uncooled THz imagers such as resistive self-mixing sensors
and bolometric sensors (based on resistive bolometers) do not
provide a low enough Noise Equivalent Temperature Difference (NETD)
for enabling passive THz imaging systems. Imagers based on coherent
heterodyne detectors, which are theoretically limited only by
quantum noise, are capable of uncooled passive imaging.
[0026] Sensors based on the resistive self-mixing approach are
limited to an NEP of approximately 66 pW/Hz.sup.1/2 which is too
large for passive imaging.
[0027] The bolometric type of sensors can be further classified
into capacitively coupled and directly coupled sensors. In the
first kind of detector an antenna is used for receiving the THz
power, which is conveyed to a termination resistor. This resistor
is physically separated from the antenna by a vacuum gap, which
also forms two capacitors (one on each edge of the resistor). The
power that is dissipated on the resistor heats up the thermally
isolated structure in which the resistor is located and the
consequent temperature shift is detected by with a sensing device.
The absence of a physical contact between the antenna and the load
is necessary for separating their thermal response.
[0028] In the case of sub-millimeter waves, the size of the antenna
causes its thermal mass to be quite large and thus the heat up time
of a pixel lacking capacitive coupling is excessively slow. Thermal
time constants longer than .about.100 msec degrade the imager's
performance when the sensors are used for real time imaging and the
read-out time is multiplexed (which is a very attractive
configuration for a sensor array). While the capacitive coupling
solution allows, in theory, better antenna performance (i.e. big
area corresponds to large gain) and good thermal performance, its
implementation with a batch micromachining (MEMS) process is rather
complex. Tolerances in the pixel geometry due to residual
mechanical stress cause a large uncertainty on the final position
of the elements, including the capacitor plates. As a consequence,
realistically achievable values of the capacitance provide a
bottleneck for the coupling efficiency and hence for the sensor's
sensitivity. Relatively good performance can still be achieved with
this solution provided that sophisticated technical solutions are
adopted resulting in higher fabrication complexity.
[0029] Directly coupled sensors address the same problem with the
opposite approach. This method requires using tiny antennas with a
small thermal mass that are turned by micromachining into suspended
and thermally isolated structures. Their implementation is feasible
for CMOS compatible bulk micromachining, which is a relatively low
cost batch fabrication process. The electromagnetic performance of
the antennas is, however, poorer than their larger mass
counterparts. Here, the need to limit the thermal mass of the
entire pixel (i.e. the antenna and the sensing
resistor/transistor/diode) forces the use of smaller sensing
devices, which are prone to higher electrical noise.
[0030] A diagram illustrating a first example embodiment cross
dipole antenna incorporating a thermal sensor is shown in FIG. 2.
The detector, generally referenced 20, comprises a cross dipole
antenna element 22, load impedance 24, thermal sensor 26 placed on
the side of the antenna and holding arm 28 connected to the
substrate 29. The inset of the sensor comprises an NMOS transistor
with W/L=3.6 .mu.m/0.36 .mu.m directly connected to the holding
arm.
[0031] As opposed to an implementation of a directly coupled
sensor, were a small temperature sensing device (e.g., transistor)
26 is placed on the side of the antenna 24, a detector is provided
incorporating a device located underneath any straight segment of
the antenna. In one embodiment, the size of the device (e.g., the
transistor's width) is the longest that fits beneath the antenna.
In another embodiment, multiple devices, placed under different
segments, are connected in parallel (or series) thereby increasing
the effective device size.
[0032] A diagram illustrating an example layout of an NMOS
transistor where the arm of a dipole antenna overlaps the gate of
the transistor over its entire length is shown in FIG. 3. The
layout, generally referenced 40, comprises a metal antenna arm 42
that lies over a long transistor. The transistor comprises a gate
44 that lies under the metal arm, drain connection 49, source
connection 46 and body (bulk) connection 48. The transistor thus
runs substantially the entire length of the antenna arm. This
alleviates the problem caused by placing large devices on the side
of the antenna in that the additional area taken up by the sensor
transistor translates to a large additional thermal mass that must
be heated by the radiation signal. By placing the sensor below the
antenna, a considerably smaller "area penalty" is paid. Such a
solution allows boosting the responsivity of the pixel while still
retaining an acceptable level of noise and demanding a dramatically
smaller increase in the thermal time constant.
[0033] A diagram illustrating a second example embodiment cross
dipole antenna incorporating a thermal sensor located underneath a
dipole arm is shown in FIG. 4. The detector, generally referenced
50, comprises an antenna arm 54, load impedances 52 and holding arm
62. The antenna arm comprises a metal wire dipole antenna 56 and
thermal sense transistor including source and drain 58 and gate 60.
The holding arm comprises conductors 64 and 66 for carrying the
readout signal to external circuitry. The thermal sensor transistor
for detecting the change in heat of the load impedance is located
directly beneath the antenna arm. A pair of NMOS transistors with
W/L=98 .mu.m/0.2 .mu.m are connected through the antenna arms and
load.
[0034] The thermal sensor that is used for detecting temperature
variations in directly coupled pixels may comprise any electrical
device whose I-V characteristic is temperature dependent. For
example, the thermal sensor may comprise one or more transistors,
diodes and resistors. A transistor is considered to be a good
choice since it provides high temperature responsivity (up to
8%/.degree. C.), which translates to large current or voltage
responsivity, even when it is biased with low static power.
[0035] Note that the bias current applied to the sensor transistor
should be sufficiently large to yield large responsivity. The bias
voltage is applied through the wires 64, 66 in the holding arm 62.
This can be achieved by increasing the bias voltage and/or by
increasing the size of the device. The latter option is preferable
since a larger device size exhibits reduced noise power spectral
density (PSD), whereas a higher gate voltage would raise the noise
level for any given device size. Note also that in other
implementations the size of the sensing transistor is severely
constrained by the pixel's response time requirements (i.e. the
thermal time constant). It is thus preferable to have a large
transistor whose area approximately overlaps the antenna's area,
thus not requiring a large amount of additional area.
[0036] The performance of the detector implementation of FIG. 2 and
that of FIG. 4 is presented in Table 1 below.
TABLE-US-00001 TABLE 1 Comparison of the detector of FIG. 2
(thermal sensor on the side of the antenna) and FIG. 4 (thermal
sensor underneath the antenna arm). Noise current Transistor size
Time PSD at W/L constant T I.sub.DS = 1 .mu.A FIG. 2 3.6 .mu.m/0.36
.mu.m 160 msec 93 pA/Hz.sup.1/2 (small transistor) FIG. 4 2 .times.
98 .mu.m/0.2 .mu.m 276 msec 17 pA/Hz.sup.1/2 (large transistor)
[0037] The data in Table 1 shows that locating the transistor under
the antenna arm provides a huge increase in transistor area
(.about.times 30) while only causing a modest increase in the time
constant (.about.1.7).
[0038] The thermal capacitance, as well the thermal time constant,
are directly proportional to the volume of the "platform" (namely
the antenna and the sensor) as shown by the following
expressions.
C.sub.th,obj=C.sub.p,obj.rho..sub.objV.sub.obj (1)
.tau..apprxeq.R.sub.arm(C.sub.arm+C.sub.ant+C.sub.sens) (2)
[0039] where C.sub.th is the specific heat capacitance per unit
mass, .rho. the density, V the volume, R.sub.arm the heat
resistance of the holding arm. Furthermore, the noise PSD for an
equal current, while operating in the proximity of or below the
threshold voltage, scales down linearly with power. The noise
current is often described by the empirical expression given by
S I .apprxeq. KF ( V GS ) I DS .alpha. ( C ox C s ) L eff .beta. W
eff .alpha. - 1 1 f ( 3 ) ##EQU00001##
[0040] where KF is the technological flicker noise parameter,
I.sub.DS the transistor's current, C.sub.ox and C.sub.s the oxide
and surface capacitance respectively, W.sub.eff and L.sub.eff the
effective width and length of the gate, f the frequency. In the
case presented in Table 1 supra we assume that .alpha.=2, .beta.=1,
C.sub.ox.parallel.C.sub.s.apprxeq.C.sub.ox. In order to reduce
noise, either W or L may be increased at the lower bias, while
large L is more effective at high bias.
[0041] Note that care should be devoted to the electrical
connections of the antenna and the thermal sensor. For example, in
the case of a dipole antenna, if the DC carrying wires were to be
placed on the side of the device, the required area would grow
considerably. Hence, the sensor wiring preferably takes advantage
of and blends with the electrical connections of the antenna
without impacting its electromagnetic performance. This is achieved
by using the antenna "arm" under which the sensor lies as a
conductor for the DC bias. Since from the point of view of the RF
signal, the middle point of the load resistor is fixed in
potential. Thus, a DC source can be connected at that point without
changing the impedance seen by the antenna and without degrading
its RF performance. Consequently, the circuit can be closed on one
side (i.e. on the dipole's furthest edge) by the connection to the
source pin 51 of the transistor, and on the other side by the
voltage supplied to the middle of the load resistance 52.
Furthermore, the parasitic resistance across the holding arm
provides RF choking towards the read out circuit, isolating the RF
from the DC signal. The voltage drop on this portion of the
resistor is a parasitic effect. This, however, is typically
negligible since the termination resistors are typically much
smaller than 1 k.OMEGA. and the bias currents are on the order of
microamperes. The arrows in FIG. 4 highlight the DC bias current
path through the holding arm wires, paths 68, 70, one antenna arm
54 and load impedance 52. Note that this approach is significantly
different from implementations of directly coupled pixels were the
antenna and the thermal sensor are coupled thermally but not
electrically.
[0042] A perspective view of a portion of the detector of FIG. 4 is
shown in FIG. 5A. The detector, generally referenced 80, comprises
antenna arm 84 (partially shown) connected to load impedance 82,
and holding arm 92. The holding arm comprises readout and bias
wires 94, 96. Antenna arm comprises dipole wire 86 and transistor
90 that lies thereunder, including gate 88.
[0043] A side view of a portion of the detector of FIG. 4 is shown
in FIG. 5B. The various layers shown include silicon oxide 100,
active silicon 102, polysilicon 104, tungsten vias 106 and copper
metal layer 108. The polysilicon gate layer 104 of the sense
transistor runs underneath along the length of the metal antenna
arm 108.
[0044] A diagram illustrating the end portion of a dipole arm
located over the thermal sensor transistor is shown in FIG. 6A. The
portion of the antenna arm shown, generally referenced 110,
comprises a transistor 112, including drain and source sides,
source contact 114, polysilicon gate 118, body contact 116 and
metal dipole arm 119. The gate of the sensor transistor runs
underneath the metal dipole arm.
[0045] A perspective view of the end portion of a dipole arm
located over the thermal sensor transistor is shown in FIG. 6B.
Shown in perspective view of the arm, generally referenced 110, are
the source and drain 112 of the sensor transistor source contact
114, polysilicon gate 118, body contact 116 and metal dipole arm
119.
[0046] A diagram illustrating the electrical model of the detector
of FIG. 4 is shown in FIG. 7A. The model depicts the equivalent
components of the detector, generally referenced 120, including
load impedances 124, thermal sense transistors 130, metal dipole
arms 122, and holding arm impedances 126 connected to VDD and 128
connected to ground.
[0047] The equivalent schematic diagram of the detector of FIG. 4
is shown in FIG. 7B. The equivalent schematic, generally referenced
140, comprises VDD source 144 connected to impedance 146, thermal
sense transistor 142 connected to voltage source 148 representing
the voltage induced by the antenna dipole elements on the load
impedance, and impedance 150. Impedances 146 and 150 are
equivalents of the holding arm wire impedances.
[0048] The terminology used herein is for the purpose of describing
particular embodiments only and is not intended to be limiting of
the invention. As used herein, the singular forms "a", "an" and
"the" are intended to include the plural forms as well, unless the
context clearly indicates otherwise. It will be further understood
that the terms "comprises" and/or "comprising," when used in this
specification, specify the presence of stated features, integers,
steps, operations, elements, and/or components, but do not preclude
the presence or addition of one or more other features, integers,
steps, operations, elements, components, and/or groups thereof.
[0049] The corresponding structures, materials, acts, and
equivalents of all means or step plus function elements in the
claims below are intended to include any structure, material, or
act for performing the function in combination with other claimed
elements as specifically claimed. The description of the present
invention has been presented for purposes of illustration and
description, but is not intended to be exhaustive or limited to the
invention in the form disclosed. As numerous modifications and
changes will readily occur to those skilled in the art, it is
intended that the invention not be limited to the limited number of
embodiments described herein. Accordingly, it will be appreciated
that all suitable variations, modifications and equivalents may be
resorted to, falling within the spirit and scope of the present
invention. The embodiments were chosen and described in order to
best explain the principles of the invention and the practical
application, and to enable others of ordinary skill in the art to
understand the invention for various embodiments with various
modifications as are suited to the particular use contemplated.
* * * * *